The invention relates to a measuring instrument for measuring the mass of a flowing medium, and particularly for measuring the rate of air flow to the air intake of an internal combustion engine.
A device for measuring the mass of a flowing medium of the above type is disclosed in DE 29 11 631 C2. Therein, a heating wire functioning as a temperature-dependent resistance is mounted together with another temperature-dependent resistance on a probe ring projecting in a tubular unit and over a cross-piece to an insert unit. The tubular unit forms the flow cross section, for example, of an intake pipe of a combustion engine. An electrical control unit, which is configured as a hybrid board circuit is adhered onto an assembly plate, in the insert unit. Conductive paths or tracks are formed on a carrier plate by thick-film technology. An increased current is passed to the heating wire after a specific measurement cycle, in order to remove any deposition formed on the heating wire.
DE 38 44 354 A1 disclosed another measuring instrument of the above type. Therein, a ceramic substrate with at least one temperature-dependent resistance layer is mounted on one side of a measuring unit and projects in an air flow passage. This measuring unit is introduced into a flow cross section, such that the flowing medium flows through the flow passage.
In order to avoid a disruption of the ceramic substrate, support elements and protective cross-pieces are provided in the flow passage.
DE 43 08 227 A1 discloses a measuring element for determining the mass of a flowing medium. Therein, a substrate is secured at one side, and a measuring resistance, a heating resistance, and a compensation resistance are disposed on this substrate.
An object of the invention is to provide a measuring instrument of the above type, which has a simple and robust structure, and an increased insensitivity to contamination.
In accordance with the invention, the measuring element is mounted in the flow passage of a housing such that it is fixed or clamped in the housing on all sides, whereby the flowing medium is guided to flow on one side at or above the measuring element. By embedding the measuring element in the housing, the measuring element is protected from breakage and consequent disruption of service. It also has a robust but simple structure. Because it is fixed in the housing on all sides, the measuring element is also relatively free of stress. The measuring element of this measuring instrument is also protected against the medium flow, since the measuring element is flush with the flow passage of the housing, so that the medium flow cannot attack the measuring element itself.
A cooling unit is conventionally mounted on the measuring element on which sensors are disposed and heat transfer to the ambient atmosphere can be obtained in the direct vicinity of the sensors. It is also advantageous that the measuring element and an electronic component for the necessary evaluation and control of the instrument can be introduced inside this measuring instrument. For this purpose, both the measuring element as well as the electronic component, which are both produced by hybrid board or silicon technology, are integrated adjacent to one another, i.e., in one plane, in the instrument. The sensors of the measuring element are protected from contamination, for example, by being embedded in membranes, which are mounted on a substrate of the measuring element. A measuring element with the utilization of membrane elements on a substrate is disclosed in copending application 09/258,865 filed concurrently herewith and claiming the priority of German Application No. 198 08 249.5.
Both the quantity of the flowing medium as well as its flow direction can be determined with only one measuring instrument, due to the arrangement of two heating sensors and two temperature sensors on one measuring element. The detection of small residual flows of medium is also possible. Both the heating sensors and temperature sensors are respectively accommodated in inside two separate membranes on one substrate.
By providing a frame around the sensor carrier, i.e., around the measuring element, an optimal flow channel geometry is produced, whereby a constriction is set up in the flow passage.